The present invention relates generally to a method and apparatus for preparing thermal adhesives such as thermoplastic, hot melt adhesives for application, and more particularly, to a method and apparatus for creating a thermal barrier in a hot melt adhesive heater or melter for the purpose of controlling and limiting the temperature rise of the adhesive in such melter. This minimizes degradation of the adhesives as a result of exposure to high temperatures for extended periods of time.
Thermal adhesives, and particularly those commonly referred to as hot melt adhesives, have been around for many years and are useful in a variety of applications. Hot melt adhesive stock is a solid or highly viscous material at room temperature and is normally provided in various forms such as pellets, granules or the like. Application of the adhesive involves heating the adhesive to an application temperature, during which the adhesive material changes from a highly viscous or solid state to a less viscous or liquid state, and then applying the same while in the less viscous or liquid state at the application temperature. As the adhesive cools, it hardens and exhibits its adhesive properties. While the adhesive material is at its application temperature, it is subject to degradation or charring. In general, minimal degradation occurs as a result of heating the adhesives to the application temperature and maintaining it at that temperature for a relatively short period of time. However, maintaining the adhesive at the application temperature for significant periods of time (generally greater than ten minutes or more) will result in dramatically increased degradation of the materials, thereby resulting in increased incidence of bond failures. Maintaining certain adhesives at the application temperature for extended periods of time can also result in the release of undesirable or dangerous fumes or other pollutants.
There are two types of adhesive melters which are currently utilized. One is a single stage melter while the other is a zone melter. A single stage melter consists of a single melting chamber having a single temperature control. During operation, the heaters associated with the single stage melter maintain the adhesive within the melting chamber at the application temperature at all times. As adhesive is drawn off for application, new, solid or highly viscous adhesive is added to the melting chamber. This new material settles to the bottom of the melting chamber because of its greater density, which is where the materials are drawn from the chamber for application. This creates generally wide temperature ranges of adhesive being drawn off for application, thereby resulting in weak or failed bonds. Also, if the application of the adhesive is stopped or delayed for an extended period of time, the adhesive in the melting chamber continues to be maintained at the application temperature and undesirable degradation or polymerization occurs.
In a multiple zone melter, an improvement is achieved by providing a high melt zone in which the temperature of the adhesive in such zone is maintained at the application temperature and a low melt or premelt zone in which the adhesive is maintained at a temperature approximately 50.degree.-150.degree. lower than the application temperature. In zone melters of this type, newly added adhesive does not go directly into the high melt zone, but is first exposed to the low melt zone. The heaters in the low melt zone function to premelt the newly added material prior to entry into the high melt zone, thereby stabilizing the temperature therein. Such a zone melter also reduces the amount of material exposed to the application temperature.
The above described zone melter is an acceptable device for stabilizing temperatures of the adhesive in the high melt zone. Such a device also reduces potential degradation of the adhesives (over that of a single stage melter because of the smaller volume of the high melt zone) while adhesive is being generally continuously applied. In such a situation, adhesive in the high melt zone is continuously drawn from that zone for use in application and adhesive from the low melt zone is continuously being introduced into the high melt zone. Thus, the amount of time which the adhesive is exposed to the application temperature when adhesive is being applied is limited.
Significant problems have also been encountered, however, with zone melters when application is terminated. In such a case, the heat from the high melt zone migrates upwardly through the low melt zone and ultimately to the entire feed hopper so that all of the adhesive in the unit is at or close to the application temperature. In many cases this will occur if the unit sits idle and is not used for as little as 10-15 minutes. Thus, any interruption of any significant time period in the use of a zone melter such as overnight, during work breaks, etc., results in the heat from the high melt zone migrating up through the entire unit. Similar to the single stage melters, this will also result in undesirable degradation or polymerization.
The above problem can be solved, to some extent, by turning off the melter during these down times or reducing the temperature control for the high melt zone to a lower temperature, however, this has a disadvantage in that the unit is not ready to go at all times. For example, if the unit is turned off completely, it could take up to an hour or more before the unit is again ready to operate. Time is also needed, if the temperature in the high melt zone is lowered, in order to increase the temperature back up to the application temperature.
Another attempt at solving the above problem has been to position the low melt zone to one side of, or laterally, with respect to the high melt zone or to physically separate the high melt zone from the low melt zone so that a separate mechanism or conduit is utilized to transfer adhesive from the low melt zone to the high melt zone. Although this has the effect of minimizing or eliminating the migration of heat from the high melt zone into the low melt zone and feed hopper, such a structure requires much more space, is more complicated and expensive and is therefore generally undesirable.
Accordingly, there is a need in the art for a hot melt adhesive heater or melter in which the various temperature zones are positioned vertically with respect to one another and in which the melter can be ready to use at all times, but which also limits or precludes migration of heat from the hot melt zone into the other zones of the unit even during significant down time periods. There is a further need for an adhesive melter having a quick cool-down feature so that temperatures in the high melt zone can be quickly reduced, if desired, for certain highly degradable adhesives.